arXiv subjects

Yong-Sun Park

Publications and source records attributed to Yong-Sun Park.

12 recordsLinked to original sources

arXiv2023

Renovation of Seoul Radio Astronomy Observatory and Its First Millimeter VLBI Observations

The Seoul Radio Astronomy Observatory (SRAO) operates a 6.1-meter radio telescope on the Gwanak campus of Seoul National University. We present the efforts to reform SRAO to a Very Long Baseline Interferometry (VLBI) station, motivated by recent achievements by millimeter interferometer networks such as Event Horizon Telescope, East Asia VLBI Network, and Korean VLBI Network (KVN). For this goal, we installed a receiver that had been used in the Combined Array for Research in Millimeter-wave Astronomy and a digital backend, including an H-maser clock. The existing hardware and software were also revised, which had been dedicated only to single-dish operations. After several years of preparations and test observations in 1 and 3-millimeter bands, a fringe was successfully detected toward 3C 84 in 86 GHz in June 2022 for a baseline between SRAO and KVN Ulsan station separated by 300 km. Thanks to the dual frequency operation of the receiver, the VLBI observations will soon be extended to the 1 mm band and verify the frequency phase referencing technique between 1 and 3-millimeter bands.

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arXiv2021

PSF Deconvolution of the IFU Data and Restoration of Galaxy Stellar Kinematics

We present a performance test of the Point Spread Function deconvolution algorithm applied to astronomical Integral Field Unit (IFU) Spectroscopy data for restoration of galaxy kinematics. We deconvolve the IFU data by applying the Lucy-Richardson algorithm to the 2D image slice at each wavelength. We demonstrate that the algorithm can effectively recover the true stellar kinematics of the galaxy, by using mock IFU data with diverse combination of surface brightness profile, S/N, line-of-sight geometry and Line-Of-Sight Velocity Distribution (LOSVD). In addition, we show that the proxy of the spin parameter $λ_{R_{e}}$ can be accurately measured from the deconvolved IFU data. We apply the deconvolution algorithm to the actual SDSS-IV MaNGA IFU survey data. The 2D LOSVD, geometry and $λ_{R_{e}}$ measured from the deconvolved MaNGA IFU data exhibit noticeable difference compared to the ones measured from the original IFU data. The method can be applied to any other regular-grid IFU data to extract the PSF-deconvolved spatial information.

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arXiv2018

DOTIFS: spectrograph optical and opto-mechanical design

Devasthal Optical Telescope Integral Field Spectrograph (DOTIFS) is a new multi-Integral Field Unit (IFU) instrument, planned to be mounted on the 3.6m Devasthal optical telescope in Nainital, India. It has eight identical, fiber-fed spectrographs to disperse light coming from 16 IFUs. The spectrographs produce 2,304 spectra over a 370-740nm wavelength range simultaneously with a spectral resolution of R=1200-2400. It is composed of all-refractive, all spherical optics designed to achieve on average 26.0% throughput from the telescope to the CCD with the help of high transmission spectrograph optics, volume phase holographic grating, and graded coated e2v 2K by 4K CCD. We present the optical and opto-mechanical design of the spectrograph as well as current development status. Optics and optomechanical components for the spectrographs are being fabricated.

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arXiv2018

Simultaneous Survey of Water and Class I Methanol Masers toward Red MSX Sources

We report simultaneous single-dish surveys of 22 GHz H2O and 44 and 95 GHz class I CH3OH masers toward 299 Red MSX Sources in the protostellar stage. The detection rates are 45% at 22 GHz, 28% at 44 GHz, and 23% at 95 GHz. There are 15, 53, and 51 new discoveries at 22, 44, and 95 GHz, respectively. We detect high-velocity (>30 km/s) features in 27 H2O maser sources. The 95 GHz maser emission is detected only in 44 GHz maser sources. The two transitions show strong correlations in the peak velocity, peak flux density, and isotropic maser luminosity, indicating that they are likely generated in the same sites by the same mechanisms. The 44 GHz masers have much narrower distributions than 22 GHz masers in the relative peak velocity and velocity range, while 6.7 GHz class II CH3OH masers have distributions intermediate between the two. The maser luminosity significantly correlates with the parental clump mass, while it correlates well with the bolometric luminosity of the central protostar only when data of the low-mass regime from the literature are added. Comparison with the results of previous maser surveys toward massive star-forming regions suggests that the detection rates of 22 and 44 GHz masers tend to increase as the central objects evolve. This is contrary to the trends found in low- and intermediate-mass star-forming regions. Thus the occurrence of both masers might depend on the surrounding environments as well as on the evolution of the central object.

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arXiv2015

Simultaneous observation of water and class I methanol masers toward class II methanol maser sources

We present a simultaneous single-dish survey of 22 GHz water maser and 44 GHz and 95 GHz class I methanol masers toward 77 6.7 GHz class II methanol maser sources, which were selected from the Arecibo methanol maser Galactic plane survey (AMGPS) catalog.Water maser emission is detected in 39 (51%) sources, of which 15 are new detections. Methanol maser emission at 44 GHz and 95 GHz is found in 25 (32%) and 19 (25%) sources, of which 21 and 13 sources are newly detected, respectively. We find 4 high-velocity (> 30 km/s) water maser sources, including 3 dominant blue- or redshifted outflows.The 95 GHz masers always appear with the 44 GHz maser emission. They are strongly correlated with 44 GHz masers in velocity, flux density, and luminosity, while they are not correlated with either water or 6.7 GHz class II methanol masers. The average peak flux density ratio of 95 GHz to 44 GHz masers is close to unity, which is two times higher than previous estimates. The flux densities of class I methanol masers are more closely correlated with the associated BGPS core mass than those of water or class II methanol masers. Using the large velocity gradient (LVG) model and assuming unsaturated class I methanol maser emission, we derive the fractional abundance of methanol to be in a range of 4.2*10^-8 to 2.3*10^-6, with a median value of 3.3\pm2.7*10^-7.

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arXiv2014

Optical Multi-Channel Intensity Interferometry - or: How To Resolve O-Stars in the Magellanic Clouds

Intensity interferometry, based on the Hanbury Brown-Twiss effect, is a simple and inexpensive method for optical interferometry at microarcsecond angular resolutions; its use in astronomy was abandoned in the 1970s because of low sensitivity. Motivated by recent technical developments, we argue that the sensitivity of large modern intensity interferometers can be improved by factors up to approximately 25,000, corresponding to 11 photometric magnitudes, compared to the pioneering Narrabri Stellar Interferometer. This is made possible by (i) using avalanche photodiodes (APD) as light detectors, (ii) distributing the light received from the source over multiple independent spectral channels, and (iii) use of arrays composed of multiple large light collectors. Our approach permits the construction of large (with baselines ranging from few kilometers to intercontinental distances) optical interferometers at the cost of (very) long-baseline radio interferometers. Realistic intensity interferometer designs are able to achieve limiting R-band magnitudes as good as ~14, sufficient for spatially resolved observations of main-sequence O-type stars in the Magellanic Clouds. Multi-channel intensity interferometers can address a wide variety of science cases: (i) linear radii, effective temperatures, and luminosities of stars; (ii) mass-radius relationships of compact stellar remnants; (iii) stellar rotation; (iv) stellar convection and the interaction of stellar photospheres and magnetic fields; (v) the structure and evolution of multiple stars; (vi) direct measurements of interstellar distances; (vii) the physics of gas accretion onto supermassive black holes; and (viii) calibration of amplitude interferometers by providing a sample of calibrator stars.

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arXiv2014

A PDR model for the FIR mid-$J$ CO ladder with universal rotational temperature in star forming region

A photon dominated region (PDR) is one of the leading candidate mechanisms for the origin of the warm CO gas with near universal $\sim$300~K rotational temperature inferred from the CO emission detected towards embedded protostars by Herschel/PACS. We have developed a PDR model in general coordinates, where we can use the most adequate coordinate system for an embedded protostar having outflow cavity walls, to solve chemistry and gas energetics self-consistently for given UV radiation fields with different spectral shapes. Simple 1D tests and applications show that FIR mid-$J$ ($14\leq J\leq 24$) CO lines are emitted from near the surface of a dense region exposed to high UV fluxes. We apply our model to HH46 and find the UV-heated outflow cavity wall can reproduce the mid-$J$, CO transitions observed by Herschel/PACS. A model with UV radiation corresponding to a blackbody of 10,000~K results in the rotational temperature lower than 300~K, while models with the Draine interstellar radiation field and the 15,000 K blackbody radiation field predict the rotational temperature similar to the observed one.

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arXiv2013

Water and Methanol Maser Survey of Protostars in the Orion Molecular Cloud Complex

The results of a maser survey toward ninety-nine protostars in the Orion molecular cloud complex are presented. The target sources are low-mass protostars identified from infrared observations. Single-dish observations were carried out in the water maser line at 22 GHz and the methanol class I maser lines at 44, 95, and 133 GHz. Most of the detected sources were mapped to determine the source positions. Five water maser sources were detected, and they are excited by HH 1-2 VLA 3, HH 1-2 VLA 1, L1641N MM1/3, NGC 2071 IRS 1/3, and an object in the OMC 3 region. The water masers showed significant variability in intensity and velocity with time scales of a month or shorter. Four methanol emission sources were detected, and those in the OMC 2 FIR 3/4 and L1641N MM1/3 regions are probably masers. The methanol emission from the other two sources in the NGC 2071 IRS 1-3 and V380 Ori NE regions are probably thermal. For the water masers, the number of detections per protostar in the survey region is about 2%, which suggests that the water masers of low-mass protostars are rarely detectable. The methanol class I maser of low-mass protostars is an even rarer phenomenon, with a detection rate much smaller than 1%.

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arXiv2007

The Far-Infrared Surveyor (FIS) for AKARI

The Far-Infrared Surveyor (FIS) is one of two focal plane instruments on the AKARI satellite. FIS has four photometric bands at 65, 90, 140, and 160 um, and uses two kinds of array detectors. The FIS arrays and optics are designed to sweep the sky with high spatial resolution and redundancy. The actual scan width is more than eight arcmin, and the pixel pitch is matches the diffraction limit of the telescope. Derived point spread functions (PSFs) from observations of asteroids are similar to the optical model. Significant excesses, however, are clearly seen around tails of the PSFs, whose contributions are about 30% of the total power. All FIS functions are operating well in orbit, and its performance meets the laboratory characterizations, except for the two longer wavelength bands, which are not performing as well as characterized. Furthermore, the FIS has a spectroscopic capability using a Fourier transform spectrometer (FTS). Because the FTS takes advantage of the optics and detectors of the photometer, it can simultaneously make a spectral map. This paper summarizes the in-flight technical and operational performance of the FIS.

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arXiv2007

The Far-Infrared Properties of Spatially Resolved AKARI Observations

We present the spatially resolved observations of IRAS sources from the Japanese infrared astronomy satellite AKARI All-Sky Survey during the performance verification (PV) phase of the mission. We extracted reliable point sources matched with IRAS point source catalogue. By comparing IRAS and AKARI fluxes, we found that the flux measurements of some IRAS sources could have been over or underestimated and affected by the local background rather than the global background. We also found possible candidates for new AKARI sources and confirmed that AKARI observations resolved IRAS sources into multiple sources. All-Sky Survey observations are expected to verify the accuracies of IRAS flux measurements and to find new extragalactic point sources.

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arXiv2007

Probing Inward Motions in Starless Cores Using The HCN J = 1-0 Hyperfine Transitions : A Pointing Survey Toward Central Regions

We have carried out a survey toward the central regions of 85 starless cores in HCN J = 1-0 to study inward motions in the cores. Sixty-four cores were detected with HCN lines. The infall asymmetry in the HCN spectra is found to be more prevalent, and more prominent than in any other previously used infall tracers such as CS J = 2-1, DCO+ J = 2-1, and N2H+ J = 1-0. We found close relation between the intensities of the HCN and N2H+ lines. This implies that the HCN is not much depleted in the central regions of the cores. In some cores, the HCN spectra show different sign of asymmetry from other molecular lines. A few cores show various signs of asymmetry in individual HCN hyperfine lines. The distribution of the velocity shift dV of the HCN profiles with respect to the systemic velocity of the optically thin tracer is found to be more shifted toward bluer side than those of other infall tracers, indicating that the HCN traces inward motions more frequently. The dV distribution of each HCN hyperfine line for all sources is similar. Moreover the dV values obtained from different HCN hyperfine lines for each source are nearly similar. These may mean that most of starless cores are in similar kinematic states across the layers of the cores. We identify 17 infall candidates using all available indicators such as the velocity shift dV and the blue to red peak intensity ratio of double peaked profiles for HCN J = 1-0, CS J = 2-1, J = 3-2, DCO+ J = 2-1, and N2H+ J = 1-0. Four of them, L63, L492, L694-2, and L1197 are found to show higher blue to red ratio in the HCN hyperfine line along the lower opacity, suggesting that infall speed becomes higher toward the center.

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arXiv2007

Velocity distribution of collapsing starless cores, L694-2 and L1197

In an attempt to understand the dynamics of collapsing starless cores, we have onducted a detailed investigation of the velocity fields of two collapsing cores, L694-2 and L1197, with high spatial resolution HCN J=1-0 maps and Monte Carlo radiative transfer alculation. It is found that infall motion is most active in the middle and outer layers outside the central density-flat region, while both the central and outermost parts of the cores are static or exhibit slower motion. Their peak velocities are 0.28 km s^{-1} for L694-2 and 0.20 km s^{-1$ for L1197, which could not be found in simple models. These velocity fields are roughly consistent with the gravitational collapse models of the isothermal core; However, the velocity gradients inside the peak velocity position are steeper than those of the models. Our results also show that the density distributions are ~ r^{-2.5} and ~ r^{-1.5} in the outer part for L694-2 and L1197, respectively. HCN abundance relative to H_2 is spatially almost constant in L694-2 with a value of 7.0 X 10^{-9}, while for L1197, it shows a slight inward increase from 1.7 X 10^{-9} to 3.5 X 10^{-9}.

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